Compressors are used in a wide variety of industrial and residential applications to circulate refrigerant within a refrigeration or heat pump system to provide a desired heating or cooling effect. Compressors are also used to inflate or otherwise impart a fluid force on an external object such as a tire, sprinkler system, or pneumatic tool. In any of the foregoing applications, it is desirable that a compressor provide consistent and efficient operation to ensure that the particular application (i.e., refrigeration system or pneumatic tool) functions properly. To that end, monitoring and controlling compressor performance helps ensures reliable and efficient compressor and system operation.
Scroll compressors are becoming more and more popular for use in refrigeration and heat pump applications due primarily to their capability of extremely efficient and consistent operation. Such compressors typically incorporate a pair of intermeshed spiral wraps that receive and compress a fluid. In operation, one of the spiral wraps is caused to orbit relative to the other so as to define one or more moving chambers, which progressively decrease in size as they travel from an outer suction port toward a center discharge port. As the moving chambers decrease in size, the fluid disposed therein becomes compressed prior to being expelled by the compressor through the discharge port. Typically, one of the scroll members is driven by an electrical motor disposed within an outer shell of the scroll compressor and is controlled by an external controller to regulate power to the motor. The electric motor, in conjunction with the controller, operates to drive the one scroll member via a suitable drive shaft to compress the fluid between the individual wraps upon demand.
Some scroll compressors are capable of adjusting capacity in response to fluctuating demand and generally referred to as “variable capacity” or “variable speed” scroll compressors. Variable capacity scroll compressors are adjusted through manipulation of the intermeshed spiral wraps such that the relative position between the individual wraps is varied and the volume of fluid disposed generally between each wrap is increased or decreased. Variable speed scroll compressors achieve a similar end, but do so without adjusting the relative position of the spiral wraps. The variable speed scroll compressor monitors system and/or compressor parameters and adjusts the speed of the electric motor that drives the orbiting spiral wrap accordingly. Such fluctuations in wrap speed affects the output of the compressor, and thus, varies the overall capacity.
In either of the foregoing variable scroll compressors, adjustment of the compressor capacity allows a system controller, such as a refrigeration system controller, to adjust the individual capacity of each scroll compressor to optimize the efficiency of the multiple-compressors rack system. For example, the controller is able to reduce capacity on a compressor if demand is decreasing, and thus, is able to reduce the energy consumed by the individual compressor. Such adjustments effectively tailor energy consumption for each compressor to only that which is minimally needed to run the system. Because the energy consumption of each variable scroll compressor may be varied, energy is regulated, and the overall system efficiency is improved.
In conventional refrigeration systems, a rack of scroll compressors may be grouped so as to function as a single unit and may provide a cooling effect to a plurality of refrigerators, refrigerator cases, or freezers. However, most compressors in a conventional rack are Fixed and their on/off cycling rate is limited by reliability requirements, thus reducing system efficiency. In any of the foregoing applications, the compressor bank generally includes a variable scroll compressor and at least one other fixed-capacity compressor. The capacity of the variable compressor may be adjusted to increase the system efficiency, as previously discussed, while the fixed scroll compressor includes a non-variable or fixed capacity.
A controller conventionally monitors the various refrigerated cases and determines an appropriate load for the system at a given time and sends a demand signal to the compressor rack accordingly. The demand signal instructs the variable compressor to operate at a particular output (i.e., one to one hundred percent of total capacity) and instructs the fixed compressor(s) to start up, continue, or shut down, depending on the state of the fixed scroll compressor at the time of instruction. While such controllers adequately control compressor capacity, such systems are limited to control of each individual scroll compressor and, therefore, are not capable of controlling a series of compressors linked in a parallel relationship.
While the controller may adequately instruct the variable compressor to function between zero and one hundred percent total capacity, the controller can only instruct the fixed controller to either start up or shut down, and therefore may instruct the rack to produce a higher capacity than required by the system. For example, if the requisite capacity calls for nine tons and the available compressors are a six ton variable scroll compressor, and two five ton fixed scroll compressors, the controller will instruct the variable compressor to run at one hundred percent and will instruct one of the fixed compressors to start up initially. However, at this point, the variable compressor is producing six tons and the fixed is producing five tons for a total of eleven tons. Therefore, the combination of the variable compressor at one hundred percent and the fixed compressor results in a two-ton overage, and thus, a loss in efficiency.
While the controller will eventually scale the capacity of the variable compressor so that the total output is nine tons, conventional controllers require sufficient time for the variable compressor to be scaled back, and therefore do not provide an optimum control algorithm. Because conventional controllers communicate with each compressor individually, overlap between compressor capacity occurs, and system efficiency is reduced.